Electron discharge device



P 1959 e. H. ROBERTSON 2,905,852

ELECTRON DISCHARGE DEVICE I Filed June 15, 1954 l I r a 9 A57;- 5 g I II FIG. I \1 3...,

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Ma /2; SURFACE /N V9 0U T M ICOATI/VG RES.(R) A C T INVENTOR GEORGE H.ROBERTSON ATTORNEY.

ELECTRON DISCHARGE DEVICE George H. Robertson, New Providence, N.J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Application June 15, 1954, Serial No. 436,802

'5 Claims. (Cl. 313-346) This invention relates to electron dischargedevices and more particularly to the activation of oxide coated cathodesin discharge devices wherein the control grid laterals are in directcontact with the cathode coating.

in accordance with conventional practices, cathodes for space dischargedevices frequently are prepared by coating the metallic base or coreelement with an emissive layer of one or more of the alkaline earthmetal compounds as, for example, the carbonates of barium, strontium andcalcium, which are then heated to be broken down to the oxides. Duringthe operation of the discharge devices, the oxide coatings are activatedby reduction as reducing agents in the cathode core metal diffuse ormigrate up to the emissive coating on the core and react with the oxidesto produce the free metal, such as barium and the like, necessary fordesirable cathode emission. The oxygen set free from the oxides as aresult of this reaction combines with the reducing agents and forms acathode interface layer at the boundary between the coating and thecore.

The existence of this interface sometimes results in a number ofdisadvantages in the operation of space dis charge devices. For example,the interface layers occuring between the oxide coating and the coremetal may make further reduction of the emissive layer metal oxide tofree metal more and more difficult by hindering the migrationtherethrough of the reducing agents in the cathode base. In addition toimpairing the emissivity of the cathode, the interface layer can befurther detrimental because the electrical resistance of the layer maybe much higher than that of the oxide coating itself. Thus, theinterface can impose severe limitations on low frequency signals in thesignal path.

Further, the core metal-interface layer-oxide coating system may actlike a rectifier due to the fact that it can be, in essence, a layer ofmetal and a semi-conductor having between them a layer of highresistance. Electrons in such a system preferably pass in the directionfrom semiconductor to metal, for the resistance to electrons from thecore to the coating is several times higher than that to electrons inthe opposite direction. Thus, as the interface resistance renderspassage of electrons from the core metal to the coating more difficultthe drawing of high emission currents from the cathode may be greatlyinhibited. In those cases where the cathode has to supply high peakcurrents, it becomes especially important to avoid as much as possiblethe formation of the interface layer.

A still additional disadvantage of the interface can be that it isincapable of combining with the emissive coating material and aconsequent inadhesion of the coating with respect to the core may occur.This could cause a partial, or possibly a complete, separation of theemissive coating and the cathode core with a resultant disruption in theoperation of the discharge device.

The principal objects of the present invention are to overcome theaforementioned difiiculties resulting from 2,905,852 Patented Sept. 22,1959 the cathode interface layer and to provide a reliable and efiicientmethod of supplying the emissive cathode coating with the necessaryreducing agents while substantially eliminating the cathode interface.

These and other objects of this invention are attained in accordancewith one illustrative embodiment comprising an electron discharge devicewherein the grid electrode contains a high proportion of reducingimpurities and the cathode core consists of substantially pure metal,the grid laterals being in direct contact with the emissive coating asdisclosed in application Serial No. 436,767, filed June 15, 1954, nowabandoned, by N. C. Wittwer. Ele'c tron tubes of the type disclosed inthe Wittwer application comprise a cathode having a metal core, a layerof electron emissive material upon a surface of the metal core and acontrol electrode in intimate contact with the emissive layer whereby ahigh value of transconductance is obtained due to the zero spacedrelationship of the control grid and the cathode coating. Signalsapplied between the grid and cathode core are supported by the bulkresistivity or resistance of the coating. As most of the resistancebetween the grid wires and the metallic cathode is in the region of thecoating adjacent the grid wires in these devices, a predominant portionof the input signal is developed between this region and the coatingspaces between grid wires, thus enabling grid control of the spacecurrent.

In accordance with aspects of this invention, reducing agents aresupplied by the grid wires rather than by the metallic core of thecathode. This permits the cathode core to be constructed of a highpurity metal, which advantageously may be nickel, and substantiallyeliminates the cathode interface, thereby resulting in the core having alow resistance contact with the coating in place of the priorobjectionable high resistance cathode interface layer. The resistancebetween the grid wires, which advantageously may also consist of nickelbut having therein a high proportion of reducing impurities, and thecathode is greater than that of other tubes having the control grid incontact with the cathode coating due to the grid interface layer createdby the migration of the reducing impurities to the cathode. This isespecially so since the rectifying barrier so formed offers maximumresistance when the grid is slightly negative with respect to thecathode as in normal use it would be.

In accordance with one feature of this invention, the cathode emissivecoating of a discharge device is activated by reducing agents containedin the control electrode of the device.

More specifically, in accordance with a feature of this invention thecontrol electrode, in a discharge device having its cathode and controlelectrode in direct contact with each other, consists of metalconductors having a high proportion of reducing impurities therein,which impurities migrate to the cathode to activate the emissive coatingthereon.

A further feature of this invention pertains to a discharge device inwhich the cathode coating is activated by impurities contained in thecontrol electrode rather than the cathode core, permitting the core toconsist of relatively pure metal thereby eliminating difficulties of thetype introduced by the creation of a cathode interface layer.

A complete understanding of these and other various features of thisinvention may be gained from a consideration of the following detaileddescription and the accompanying drawing in which:

Fig. 1 is a side view of one specific illustrative embodiment of a zerospaced discharge device in accordance with the instant invention, aportion of the envelope having been broken away; V

Fig. 2 is a circuit representation of a space discharge deviceillustrating the effect of the cathode interface and coating resistanceof prior art devices;

.-Fig. 3-isl-a diagram depicting the equivalent circuit forthe-structureof-Fig. 2;

Fig.4 is .a circuit representation of a space discharge device inaccordance with the embodiment of the present invention; and

;Fig. 5 is a diagram depicting .the equivalentcircuit for thesstructureQfFig. 4.

JReferringmow to the drawing, the specific illustrative embodimentofthis invention depictedin Fig. 1 comprises adischarge device includingan :envelope 14, a heater element -15,*a.cathodehavingametal coreor base.10 and u layer of electron emissive material 11 upon a surfaceof themetal-core 10, a controlelectrode comprising a pluralityof metallicconductors or grid laterals 12 in intimate contact with the layer-ofemissive material 11 andan anode 13. The metallic core mayadvantageously consist of highpurity nickel such as that having oftheorder'of .01 percent total impurity or less; the Grade A nickel employedby many manfacturers has approximately onepercent total impurities andis not sufiiciently pure to eliminatesat-isfactorily undesirable cathodeinterface effects. Further, the cathode is advantageously held in.positionby supports substantially free of reduction impurities; suchsupports may be of very pure nickel, ceramics, molybdenum and the like,so that reducing agents cannotditiuse .or migrate from the cathodesupports into the cathode base metal and contaminate the same.

The .electron emissive layer 11, which includes a reducibleelectron-producing metal compound, may be applied to the surface of themetallic base 10 in any known manner as by spraying or dipping and mayadvantageously comprise one of the alkaline earth metal compounds suchas the oxides of barium, strontium or calcium, or a mix ture of two ormore of the same.

.In accordance with an aspect of this invention the activatingagents forthese oxides are contained in the control electrode conductors 12 whichmay advantageously consist of fine nickel wires made from nickel-havinga high proportion of such reducing agents therein. This method may haveup to the order of one percent or more impurities, of-which about a halfare reducing agents, depending on the operating conditions of the tube,particularly the temperature of the cathode. These reducing agents maybe silicon, carbon, magnesium, aluminum and .other metals of the typewhich will react with the oxides of the emissive coatingtoprovide thefree barium .or other oxide metals contained in the coating. Thereducing agents migrate through the electrode conductors .12on heatingof the cathode coating by the heater element .1-5 or in other ways knownin the art, to attain thermionic emission from the coating. The anode 13is conventional and may be constructed in any manner known in the art.Manifestly, the principles of the instant invention are not limited intheir application to particular electron tube configurations andmayadvantageously be utilized in a wide variety of multielectrode tubessuch as .tetrodes, pentodes, and the like.

Figs. 2 and 3 are the schematic diagram and the equivalent circuitrespectively of a discharge device of the type wherein the reducingagents are supplied by the cathode base, in accordance with the priorart, and illustrate the efiect of the cathode interface layer.

In Fig. 2, V is the input signal voltage applied between the ,grid andground and V is the voltage drop across the combined resistance of thecathode interface layer and the emissive coating, which may be referredto as the feedback voltage. As can be seenfrom the equivalent circuitdepicted in Fig. 3, the voltage drop V across the combined resistancewill be equal 'to the plate signal current i times the combined cathoderesisted? Rc+ b written as: v

4 k= p( i+ c) (1) This means that if the interface resistance R, ishigh, not only is a large proportion of the output signal voltagedissipated across the interface layer and thus not available to theoutput load circuit, but the value of the output generator electromotiveforce'is greatly reduced by the component i R, of the feedback voltage VHowever,-in a-discharge'device in accordance with thepresent-.invention,.as .shown schematically in Fig. 4 andinequivzilentcircuit formin Fig. 15, the interface layer is built up atthe boundary of'the-cathode surface and the control grid and is nottherefore in the output signal path. As shown in Figs. 4 and *5, theoutput signal voltage drop V racross thecathode-isthe product of theplate current i and only the coating resistance R which voltage may bewritten as:

The grid interface resistance is now in the input circuit, and as longas it is substantially greater than, i.e., in the order of ten times orrnore, the loading resistance on the input circuit it will have nodeleterious effect on the operation of the discharge device.

It is desirable that the ratio of the grid interface resistance R, I tothe cathode coating resistance R, .be relatively high in order that mostof the-input voltage V appear between the grid and cathode surface.Advantageously,.this can be done by including in the grid wiresimpurities, such as silicon and the like, which are known .to form highresistance interfaces between the wires and the cathode coating. Inaddition, the fact that the area of contact between the grid wires andthe cathode coating is small compared with the area of the cathode helps'to provide the desired high ratio of interface to cathode coatingresistance. A still further means of keeping this ratio high is toprovide a low value of cathode coating resistance such as by operatingwith an excess of free barium .in the coating. This will have theadditional advantage of keeping the feedback voltage -V .to a'smallvalue.

The advantages of the instant invention are then manifest because inaddition to the elimination of the difficulties recited heretofore, theequivalent circuits of :Figs. 3 and 5 and the'Equations land Zmake itclear that in order to ;get the same output voltage across a given loada higher value of input signal voltage V is needed 'for the tube with:cathode interface than'for the tubezin which this has been eliminated.

Thus, I have shownand described a specific illustrative embodiment of ahigh transconductance zero spaced discharge device in which the reducingagents for the emissive .coating of the cathode :areobtained from thegrid electrode rather-than from the "cathode base metal. It is to beunderstood that the above-described arrangements are but illustrative ofthe application of the principles of this invention'and' that otherarrangements may be made by'those skilled the :art without'departingfrom the spirit and .scope :of this.invention.

What isclaimedis:

1. An-electron discharge device comprising a cathode, an anode, and.acontrol electrode for controlling-electron emission from saidzcathode inresponse to applied signals,-said cathode'comprising a core, an emissivelayer on. said Jcore including as an element thereof a reducibleelectron producing metal compound, said COlIltIOlBi6C- trode being indirect contact with said emissive layer, said core having an.insuflicient amount of reducing agents therein toreduce said compoundappreciably, and means for activating :said emissive layer comprisingaplurality of reducing agents in the material .of said control electrode.

2. An electron discharge 'device comprising a cathode, an anode, and acontrol electrode for controlling electron emission from said cathode inresponse to applied signals, said'cathodecomprisingacore o'fa sub-Stantially pure refractory metal and a layer of electron emissivematerial thereon, said control electrode being in direct contact withsaid emissive layer, and means for activating said emissive layercomprising a plurality of reducing agents in the material of saidcontrol electrode.

'3. An electron discharge device in accordance with claim 2 in whichsaid cathode core is comprised of nickel.

4. An electron discharge device in accordance with claim 2 in which saidcontrol electrode comprises relatively impure nickel having a pluralityof reducing agents therein.

5. An electron discharge device in accordance with claim 1 in which saidlayer comprises at least one of the alkaline earth metal oxides.

References Cited in the file of this patent UNITED STATES PATENTSHartmann et a1. Apr. 9,

Slepian Ian. 5,

Widell Dec. 22,

Spencer Nov. 4,

Jacobs June 17,

FOREIGN PATENTS Great Britain May 21,

Germany Oct. 7,

